1. Field of the Invention
The present invention relates generally to amorphous solar cells, and more particularly, to an amorphous solar cell having a high initial photoelectric conversion efficiency and a suppressed light degradation rate.
2. Description of the Background Art
In general, an amorphous solar cell has many advantages. For example, because the amorphous solar cell has a larger absorption coefficient of light than that of a single crystal solar cell, the thickness of a silicon layer can be decreased. In addition, the manufacturing process is simple. Furthermore, because the manufacturing temperature is low, a small amount of energy is used during the manufacturing. However, in order to put the amorphous solar cell into practice as a solar cell for providing electric power, improvements must be made for the photoelectric conversion efficiency and the prevention of light degradation. As measures of preventing the above described light degradation, consideration has been given to the development of a material of an i layer which is not degraded by light or an improvement from the viewpoint of a structure produced by using a tandem cell (Appl. Phys. Lett., 35(2), July 15, 1979).
FIG. 10 is a diagram illustrating a conventional amorphous solar cell having a single cell structure.
Referring now to FIG. 10, this amorphous solar cell includes a glass substrate 23. A transparent conductive film 24, a p layer 26, an i layer 27, and an n layer 28 are successively formed on the glass substrate 23 in the above described order. The transparent conductive film 24 is formed of, for example, ITO (Indium Tin Oxide) and SnO.sub.2. The p layer 26 is formed by including p-type dopant impurities in amorphous silicon. The i layer 27 is formed of undoped amorphous silicon. The n layer 28 is formed by including n-type dopant impurities in amorphous silicon. An electrode 25 is provided on one end of the transparent conductive film 24. The electrode 25 is electrically connected to an output terminal 25a. An electrode 29 is provided on the n layer 28. The electrode 29 is electrically connected to an output terminal 29a. The electrodes 25 and 29 are formed of, for example, aluminum. Light enters the cell through the glass substrate 23 from the direction represented by the arrow.
FIG. 11 illustrates the relation between the light degradation rate and the thickness of the i layer 27 of an amorphous solar cell having a single cell structure that is obtained when the cell is irradiated by light for 8 hours under the condition of AM (Air Mass)=1.100 mW/cm.sup.2.
Referring to FIG. 11, the abscissa represents the thickness (di) of the i layer 27 and the ordinate represents amount of light degradation (1-.eta..sub.B /.eta..sub.A), where .eta..sub.A represents an initial photoelectric conversion efficiency and .eta..sub.B represents a photoelectric conversion efficiency after photoirradiation. FIG. 12 illustrates the relation between the initial photoelectric conversion efficiency (.eta..sub.A) and the thickness (di) of the i layer 27.
As is obvious from FIG. 11, when the thickness of the i layer 27 becomes 3000 .ANG. or less, the amount of light degradation (1-.eta..sub.b /.eta..sub.A) is rapidly decreased. On the other hand, as is obvious from FIG. 12, the initial photoelectric conversion efficiency (.eta..sub.A) is at a maximum when the thickness of the i layer 27 is around 6000 .ANG.. Thus, the thickness i layer 27 must be decreased so as to reduce the amount of light degradation, while the thickness of the i layer 27 must be increased so as to increase the initial photoelectric conversion efficiency. Thus, it is difficult to reduce the amount of light light degradation of the cell while holding the initial photoelectric conversion efficiency at a constant value.
FIG. 13 is a diagram illustrating a conventional amorphous solar cell having a tandem cell structure.
Referring to FIG. 13, this amorphous solar cell includes a glass substrate 30. A transparent conductive film 31, a p layer 33, an i layer 34, an n layer 35, a p layer 36, an i layer 37 and an n layer 38 are successively formed on the glass substrate 30 in the above described order. An electrode 32 is provided on one end of the transparent conductive film 31. The electrode 32 is electrically connected to an output terminal 32a. An electrode 39 is provided on the n layer 38. The electrode 39 is electrically connected to an output terminal 39a. Two cells each having a pin structure are .connected in series Light enters the cell through the glass substrate 30 from the direction represented by the arrow.
In this conventional solar cell having a tandem cell structure, the thickness of the upper i layer 34 on the side of incident light can be made small, such as approximately 1000 .ANG.. However, respective pin structures of the tandem cells are connected in series, so that the values of currents respectively generated on the upper i layer 34 and the lower i layer 37 must be equalized. Therefore, in order to obtain high light electric conversion efficiency, the thickness of the lower i layer 37 must be approximately 5500 .ANG..
FIG. 14 illustrates the relation between the thickness (di) of the lower i layer 37 and the amount of light degradation (1-.eta..sub.B /.eta..sub.A) of the amorphous solar cell having a tandem cell structure illustrated in FIG. 13 that is obtained when the cell is irradiated by light for 8 hours with the thickness of the upper i layer 34 being approximately 1000 .ANG. under the condition of AM=1.100 mW/cm.sup.2.
FIG. 15 illustrates the relation between the thickness of the lower i layer 37 and the initial photoelectric conversion efficiency (.eta..sub.A) of the amorphous solar cell having a tandem cell structure illustrated in FIG. 13 that is obtained when the thickness of the upper i layer 34 is approximately 1000 .ANG..
As is obvious from FIG. 14, when the thickness of the i layer 37 of a lower cell becomes 3000 .ANG. or more, the lower cell is degraded by the light. In this case, the thickness of the i layer 34 of an upper cell is small, approximately 1000 .ANG. so that the upper cell is not degraded by the light. The amount of light incident on the lower cell, a part of the, incident light being absorbed by the upper cell, is approximately one-half of that of the single cell. Thus, the light degradation rate of the lower cell becomes approximately one-half of that of the single cell. The light degradation rate of the tandem cell is determined by the light degradation rate of the lower cell, which is approximately one-half of the single cell.
On the other hand, as is clear from FIG. 15, the initial photoelectric conversion efficiency (.eta..sub.A) is at a maximum when the thickness of the i layer 37 of the lower cell is around 6000 .ANG..
More specifically, even in the amorphous solar cell having a tandem cell structure, the thickness of the lower i layer 37 must be decreased so as to reduce the amount of light degradation of the cell, as in the amorphous solar cell having a single cell structure. Consequently, the thickness of the lower i layer 37 must be increased so as to increase the initial photoelectric conversion efficiency. Thus, even in the amorphous solar cell having a tandem cell structure, it is difficult to reduce the amount of light degradation of the cell while maintaining the initial photoelectric conversion efficiency at a constant value.
As described above, it is difficult to reduce the amount of light degradation of the conventional amorphous solar cell while maintaining the initial photoelectric conversion efficiency whether the cell is of a single cell structure or a tandem cell structure.